The generalizability of study findings may be influenced by pre-enrollment trial procedures, including the use of behavioral run-in periods. The study goals were to determine whether behavioral run-in periods and other pre-enrollment processes affect outcomes in randomized trials of behavioral weight loss interventions that have contributed to clinical guidelines.

The sample was behavioral weight loss intervention trials included in the 2018 U.S. Preventive Services Task Force systematic review. Information on pre-enrollment processes (total steps, in-person steps, behavioral run-in) was abstracted, and meta-regressions were conducted in 2022 to test whether pre-enrollment processes were associated with weight loss at 6 or 12 months and trial retention at 12 months.

Across 80 trials, the median number of total pre-enrollment steps was 2 (range=1–5), and that of in-person pre-enrollment steps was 1 (range=0–4). Almost one-third of the trials (k=24; 30%) used a behavioral run-in. The most common run-in tasks were self-monitoring physical activity (k=9) or both physical activity and diet (k=6). Greater weight loss was observed in trials with behavioral run-ins at 6 months (−2.33 kg; 95% CI= −3.72, −0.93) and, to an attenuated extent, at 12 months (−0.86 kg; 95% CI= −1.72, 0.01) compared to those without run-ins. The total number of pre-enrollment steps was also associated with greater 6-month weight loss (−0.85 kg; 95% CI= −1.59, −0.11). Higher retention was associated with total number of pre-enrollment steps and in-person steps and marginally with the presence of run-ins.

The use of more pre-enrollment processes is associated with greater weight loss in behavioral weight loss trials and may impact the generalizability of outcomes.